1. Field of the Invention
The present invention relates to an automatic V-belt transmission which is suitable for vehicles such as four-wheeled all-terrain vehicles and two-wheeled vehicles.
2. Description of the Related Art
Conventionally, there has been provided an automatic V-belt transmission which has a belt clutch function from a view point of enhancing operability of a vehicle. In the mechanism, while an engine mounted on the vehicle is idling, the belt clutch is being disengaged so as not to cause any excessive creep phenomenon to the vehicle.
FIG. 10 shows a drive pulley 1 for the conventional automatic V-belt transmission having the belt clutch function. The drive pulley 1 has a drive shaft 10 connected to a crankshaft 8 of the engine, a fixed sheave 11 which is fixed to the drive shaft 10, a movable sheave 12 which can move in a direction of the drive shaft axis O1, a flyweight type of propulsion generating mechanism 15 for the drive pulley which is mounted behind the movable sheave 12, and so on.
The propulsion generating mechanism 15 has a flyweight 42 which expands radially outwardly due to its centrifugal force. That is, with the increase in the rotational speed of the drive shaft 10, the flyweight 42 expands outwardly, the flyweight 42 pushes a roller 44 of a spider 43 which is fixed to the drive shaft 10, and the movable sheave 12 and a spring receiving disk 38 fixed to the movable sheave 12 are integrally pushed toward the fixed sheave 11 against a biasing force of a return spring 46.
In the mechanism, in order to exert the belt clutch function while the engine is idling as shown in FIG. 10, there exists play (i.e. a clearance) between a side edge surface 3a of the V-belt 3 and a conical pressure surface 11a of the fixed sheave 11 and/or play between a side edge surface 3b of the V-belt 3 and a conical pressure surface 12a of the movable sheave 12, so that the disengagement of the clutch is maintained.
FIG. 11 is an enlarged longitudinal sectional view which shows sectional shapes of the V-belt 3, of the conical pressure surface 11a of the fixed sheave 11 of the drive pulley 1, and of the conical pressure surface 12a of the movable sheave 12 of the drive pulley 1, respectively. A pinching angle (V-angle) 2 α forming between the conical pressure surface 11a of the fixed sheave 11 and the conical pressure surface 12a of the movable sheave 12, is slightly smaller than a V-angle 2 β between both of the side edges of the V-belt 3. Also, the conical pressure surfaces 11a and 12a are formed symmetrically with respect to a plane M which is perpendicular to the drive shaft axis O1, and the conical pressure surfaces 11a and 12a are formed such that each of inclined angles α and α between the plane M and the surfaces 11a and 12a is formed to be constant, from their radially inner part to their radially outer part.
According to the automatic V-belt transmission having the belt clutch function as shown in FIG. 10, a self-induced vibration (or self-excited oscillation) occurs in a circumferential direction of the drive pulley 1, on the basis of mutual slide between the V-belt 3 and the fixed sheave 11 and between the V-belt 3 and the movable sheave 12, in the operational region of the belt clutch from a state of idling of rotation up to a state of maximum reduction in speed (i.e. a state of low speed) through a half-clutch state. As a result, the self-induced vibration triggers the occurrence of so-called “belt squeaking noise (or belt squeaking sound)”.
In order to reduce the noise, it is effective that the angular difference (2β−2α) between the V-angle 2 β of the V-belt 3 and the pinching angle 2 α of the drive pulley 1 in the clutch operational region is made substantially larger by making the pinching angle 2 α, shown in FIG. 11, smaller intentionally, such that offset load is exerted by the sheaves 11 and 12 about radially outer edge parts B, B of the V-belt 3, and contact pressures between the side edge surfaces 3a, 3b and the conical pressure surfaces 11a, 12a are increased.
However, in a case of biasing the contacting region of the V-belt 3 and the conical pressure surfaces 11a, 12a of the drive pulley 1 towards the radially outer edge part B by making the angular difference between the V-angle 2 β and the pinching angle 2 α large, contact pressure centers between the side edge surfaces 3a, 3b of the V-belt 3 and the conical pressure surfaces 11a, 12a of the sheaves 11, 12 may be swerved relative to each other, due to the variation, caused upon manufacturing, in each of inclined angles β forming the V-angle 2 β of the V-belt 3, and due to the variation in each of the inclined angles α of the conical pressure surfaces 11a and 12a of the sheaves 11 and 12. As a result, there arises a moment which twists the V-belt 3 in directions shown by reference characters A and A in FIG. 11.
In the clutch operational region in which the V-belt 3 slowly moves over the conical pressure surfaces 11a and 12a while sliding over the surfaces 11a and 12a, the contact pressure centers of both of the side edge surfaces 3a and 3b of the V-belt 3 subtly change, and the direction of the aforementioned twisting moment changes. Namely, the V-belt 3 is transferred over the conical pressure surfaces 11a and 12a while causing the twisting vibration, the characteristics of the friction being exerted therebetween changes, and the self-induced vibration having a relatively long period, or so-called “judder phenomenon”, easily occurs.
On the other hand, Japanese Laid-Open Patent Publication No. 8-326859 discloses an automatic V-belt transmission, in which the inclined angle of the radially inner peripheral side is smaller than the inclined angle of the radially outer peripheral side by making the conical pressure surface of the drive pulley be double-angled. The mechanism intends to prevent the belt squeaking noise by making both of the sheaves on the fixed side and the movable side be double-angled symmetrically. According to the mechanism, it is possible to prevent the generation of the squeaking noise of the belt in the clutch operational region. However, it is not possible to suppress the judder phenomenon of the belt due to the generation of the twisting moment.